The prior art for this kind of frequency sweep measuring unit will be described hereinbelow with reference to a spectrum analyzer as an example in order to elucidate issues.
FIG. 1 shows the principle and construction of a conventional spectrum analyzer, and FIG. 2 shows a specific example of construction of an IF assembly 50 shown in FIG. 1.
The spectrum analyzer shown in FIG. 1 comprises three channels of frequency converters, and a range of frequencies which are to be measured is divided into three bands including a low band Flow, a high band Fhi and a super-high band RFin, as shown in FIG. 3. In terms of specific figures, the low band Flow ranges from 0.1 MHz˜3.6 GHz, the high band Fhi ranges from 3.6 GHz˜8 GHz, and the super-high band RFin ranges from 8 GHz and higher.
A signal being measured in the low band Flow and in the high band Fhi of 0.1 MHz˜8 GHz which is input from a first input terminal T1 is attenuated to a given range of levels in an input attenuator 10, and subsequently a change-over switch SW1 is changed to feed it through a low pass filter 12 to a mixer 14 during the measurement of the low band Flow and to feed it through a variable tuning filter 16 to a mixer 18 during the measurement of the high band Fhi.
When the signal being measured is in the low band Flow of 0.1 MHz˜3.6 GHz, an output from the mixer 14 is passed through a band-pass filter BPF 24 to be fed to a mixer 28 where it is converted into an intermediate frequency signal having an intermediate frequency Fi by a local signal from a fixed oscillator 26. Specifically, after frequency components of the signal being measured which are equal to or below 3.6 GHz are passed through and delivered from the low pass filter 12 which comprises a preselector, they are subject to an up-conversion in the mixer 14 by a frequency sweep signal from a frequency sweep oscillator 20. Assuming that an output frequency from the mixer 14 is 4.2 GHz, a sweep frequency Flo of the sweep oscillator 20 sweeps frequencies 4.2 GHz˜7.8 GHz. The band-pass filter 24 only passes 4.2 GHz component. When the local signal from the fixed oscillator 26 has a frequency of 3.8 GHz, there is obtained a signal having an intermediate frequency of 0.4 GHz from the mixer 28. This intermediate frequency signal is fed through a change-over switch SW3 to an IF assembly 50.
When the signal being measured is in the high band Fhi of 3.6 GHz˜8 GHz, an output from the YIG variable tuning filter 16 is subject to a frequency mixing with a sweep signal from the sweep oscillator 20 in the mixer 18, and an output from the mixer 18 is passed through a low pass filter 30 to derive a component having an intermediate frequency of 0.4 GHz. Thus, the sweep frequency Flo of the sweep oscillator 20 sweeps frequencies in 4.0 GHz˜8.4 GHz. In interlocked relationship with the sweep, the tuning frequency of the variable tuning filter 16 is varied in order to feed only a frequency component of the signal being measured which is to be measured to the mixer 18, and only a component of the output signal from the mixer which has an intermediate frequency Fi of 0.4 GHz is passed through the low pass filter 30, and then fed to the IF assembly 50 by the change-over switch SW3.
The YIG variable tuning filter 16 comprises a preselector which uses YIG (Yttrium Iron Garnet), and allows only a frequency component to be measured which corresponds to the sweep frequency Flo to be passed therethrough by changing an exciting magnetic field applied to YIG so that the passed frequency tracks the sweep frequency Flo of the sweep oscillator. The YIG variable o tuning filter 16 has a power demand of several tens of watts in order to apply a desired exciting magnetic field and for other purposes. The YIG variable tuning filter 16 has a good filtering response, but is expensive and bulky.
Finally, when the signal being measured is a signal in the super-high band RFin equal to or above 8 GHz, an external mixer 200 is provided outside a casing 300 of the spectral analyzer, and an N-th harmonic wave of the sweep signal from the sweep oscillator 20 is utilized in the frequency conversion of the signal being measured. Specifically, the sweep signal as branched by a coupler 22 is fed to a buffer amplifier 32, whereby harmonics inclusive of the fundamental wave of the sweep signal is fed through a coupling capacitor 34 to the external mixer 200 as a local signal L0. In this manner, if harmonics are used up to N=7, for example, it is possible to measure frequencies as high as 50 GHz and above.
On the other hand, the signal being measured in the super-high band RFin which is input from an input terminal T3 is fed to the external mixer 200 through an external band-pass filter 202 not shown. The external band-pass filter 202 comprises a preselector, and is a fixed-mode band-pass filter having a filtering frequency response which corresponds to the measured frequency band in the super-high band RFin.
Because the super-high band RFin is very broad, each time when a measured frequency band is changed within the super-high band, there arises a need to change the external filter 202 to one which corresponds to the measured frequency band in the super-high band RFin. In other words, it is necessary that external filters 202 be provided each having a pass frequency which corresponds to each frequency band within the super-high band RFin. By way of example, if pass frequency bands are delineated in unit of 0.4 GHz and if 8 GHz˜50 GHz are subject to the measurement, there must be provided as many external filters 202 as (50 GHz˜8 GHz)/0.4 GHz=105. It is impractical to provide so many external filters 202.
There is an instance that a YIG tuning filter (YTF) which comprises a variable tuning filter is used in place of the fixed-mode external BPF 202, and a control signal is fed to cause the YIG tuning filter 204 to be tuned to the frequency of an N-th harmonic of the sweep signal which acts as a local signal. With such an arrangement, it is possible to establish each desired pass frequency band using a single variable tuning filter 204. However, it is to be noted that such YIG tuning filter 204 itself represents a very expensive component, and may cost as much as comparable to the apparatus proper.
The external mixer 200 is put to use when it is connected to an input terminal T2 on the casing 300 by a user. The signal being measured which is in the super-high band RFin is subject to a down conversion in the external mixer 200 by means of the N-th harmonic wave of the sweep signal to provide an intermediate frequency signal IF of 0.4 GHz, which is delivered to the input terminal T2, and a high suppress choke filter 36 allows only a low band component 36s including the intermediate frequency Fi to be passed therethrough to be fed to the IF assembly 50 through the change-over switch SW3.
A sweep controller 100 controls the sweep oscillator 20 so as to perform a frequency sweep across a frequency span which corresponds to the frequency converter in one of three channels including the channel using the mixer 14, the channel using the mixer 18 and the channel using external mixer 200 and which is selected by the user. A portion of the sweep controller 100 which undertakes the high band Fhi of 3.6 GHz˜8 GHz performs a tracking control of the tuning frequency of the YIG variable tuning filter 16 in interlocked relationship with the frequency sweep of the sweep signal. Where the YIG variable tuning filter 204 is used, this YIG variable tuning filter 204 is similarly subject to a tracking control.
The IF assembly 50 includes a variable bandwidth filter which allows only frequency components of the input intermediate frequency signal which are in a desired bandwidth to pass therethrough, and subsequently converts it into given digital data, which is then stored in a buffer memory for each sweep. An example of its internal construction will be described below with reference to FIG. 2.
An intermediate frequency signal from one of the frequency converters in the three channels is input, amplified in a variable gain amplifier 51 to be fed to a band-pass filter 55 where only components having an intermediate frequency Fi of 0.4 GHz are allowed to pass, and the passed output is subject to a down conversion to a lower intermediate frequency Fi2, for example, 20 MHz, in a mixer 56 by means of a local signal from a fixed oscillator 57. The lower intermediate frequency signal is band limited with a desired resolution bandwidth by an intermediate frequency filter 52 which can be set up to any bandwidth response. The band limited intermediate frequency signal is applied to a logarithmic converter 58 where its amplitude is converted into a logarithmic value. A corresponding converted output is detected by a detector 53 and the detected output is quantized into digital data in an AD converter 54, and the resulting digital data is continuously stored in a buffer memory 59 in unit of one sweep.
Returning to the description of FIG. 1, the spectrum data in unit of one sweep which is acquired in the IF assembly 50 is supplied to a display 90, thus displaying the spectrum of the signal being measured and else. The IF assembly 50, the display 90, the sweep controller 100, switches SW1, SW2, SW3 are controlled by an apparatus controller 110. When the center frequency and the frequency span for the signal which is desired to be measured is set up in the apparatus controller 110, the apparatus controller 110 controls various parts in accordance with the values set up.
As described above, a conventional spectrum analyzer has three channels of frequency converters, and in each channel of the frequency converter, there is provided a preselector separately in order to avoid a wrong measurement which may be caused by an image signal and which results from a frequency component which is spaced from the signal of the intended frequency by twice the intermediate frequency. For this reason, there has been a need to provide a number of components including a plurality of mixer components, change-over switches SW1, SW2, SW3 and the like. In particular, for the measurement of the super-high band RFin, a number of band-pass filters 202 or an expensive YIT variable tuning filter 204 has been used. This resulted in a high cost.
In addition, a high frequency circuit requires a mounting structure having shielded spaces created by metal shield constructions as may be formed by aluminum die castings, for example, between various circuit function blocks in order to prevent interferences between adjacent circuits from occurring. However, with three channels of frequency converters, there exist a number of circuit blocks, thus requiring a number of shielded spaces. As a consequence, the cost of the apparatus increases in proportion to the circuit blocks and the circuit elements. The provision of a number of shielded spaces result in a casing of an increased size, rendering it difficult to reduce the size and the weight.
A technology which overcomes such problems is proposed in Japanese Laid-Open Patent Application No. 233,875/96 (or counterpart U.S. Pat. No. 5,736,845, issued Apr. 7, 1998). This is illustrated in FIG. 4 where a signal being measured from an input terminal T3 is fed to an external mixer 200 without passing it through a preselector, and a sweep signal which represents an N-th harmonic wave of a sweep oscillator 20 is used to convert the signal being measured into an intermediate frequency Fi in the external mixer 200, and the converted signal is fed to an IF assembly 50 through a terminal T4 and a switch SW3. The IF assembly 50 feeds measured data to an image eliminator 70 which eliminates data which is based on an image signal before the data is fed to a display 90.
The elimination of image data will be described in terms of specific values. It is assumed that an intermediate frequency Fi=0.4 GHz and that a signal being measured is a single 30 GHz signal. It is also assumed that the sweep frequency of the oscillator 20 is fosc=4˜8 GHz and that the measurement takes place with N=5.
First of all, the discrimination of an image signal will be described. Denoting the frequency of a signal being measured by fs, when the intermediate frequency Fi is produced, there are produced two components having an absolute value of Fi=400 MHz=fs±(fosc×N). When the sweep takes place using a normal sweep frequency fosc and another sweep frequency fosc+2Fi which is shifted by twice the intermediate frequency Fi, the resulting signal components will be such that detected points (points representing measured frequencies) which result from the both sweeps will coincide in position only for the intended signal fs. The signal data and the image data are discriminated from each other by utilizing the coincidence of the detected points. Specifically, when the two sweep frequencies mentioned above are used with the sweep frequency fosc and the execution of the sweep is controlled in an alternate fashion, signals FN5a, FN5b are produced from the sweep with fosc, as shown in FIG. 5(A), and signals FN5c, FN5d shown in FIG. 5B are produced from the sweep with fosc+2Fi. Denoting the first sweep frequency by fosc1 and choosing the second sweep frequency such that fosc2=fosc1+(2Fi/5), it follows that a fifth harmonic wave has frequencies of 5×fosc1 and 5×fosc1+2Fi, with consequence that the second sweep frequency fosc2 sweeps with a shift just equal to the intermediate frequency 2Fi. As a consequence, the spectrum position resulting from the second sweep frequency fosc2 appears at the position of FN5c, FN5d relative to the spectrum signals FN5a, FN5b which result from the first sweep, namely, with a shift of 2Fi to the left (at the lower measured frequencies). Since the spacing between FN5a and FN5b and the spacing between FN5c and FN5d are equal to 2Fi, it exists at the position where FN5a and FN5d overlap each other, or at a same measured frequency point 30 GHz.
An image eliminator 70 eliminates an image by comparing data for each measured frequency point in a train of spectrum data (measured data) which are obtained from the alternate sweeps, and delivering data having a smaller value to the display 90. When this processing is applied to the four signals FN5a, FN5b, FN5c and FN5d mentioned above, the result will be as illustrated in FIG. 5C where it will be noted that only the signals FN5a arid FN5d which exist at an overlapping position is delivered as a single spectrum. Similarly, spectrum data (measured data) which are produced by N=4-th, 6-th and 7-th harmonic waves of the sweep frequencies of fosc1 and fosc2=fosc1+(2Fi/5) have a shift frequency of 4×(2Fi/5) or 6×(2Fi/5) or 7×(2Fi/5). It will be seen from the positional relationship of shifting by the intermediate frequency Fi×2 that four data items all assume offset positions (measured frequency points), as shown in FIGS. 5D, E and F, respectively, and accordingly they are eliminated by the image eliminator 70. As a consequence, there is obtained a spectrum indication shown in FIG. 5C, meaning that only the intended signal is obtained as a spectrum.
With the prior art mentioned above, two sweeping measurements are performed while shifting the sweep frequency by 2Fi, and an image eliminating processing is applied to the both spectrum data (measured data), thus allowing the expensive external band-pass filter 202 or variable YIG tuning filter 204 shown in FIG. 1 to be omitted.
However, a correct measurement is only possible under the condition that a signal being measured which is input as well as a spurious signal which is generated within the measuring apparatus do not appear at an image frequency point or at FN5a, FN5c as considered in FIG. 5, during the frequency sweep.
This point will be discussed with reference to FIG. 6. In order to facilitate an understanding, it is assumed that a center frequency fc is located at 5 GHz, a frequency span Fs is equal to 2 GHz or a spectrum measuring interval (a range of set-up frequencies) ranges F1=4 GHz˜F2=6 GHz, and an input signal S2 of 6.6 GHz which exists outside the range of set-up measured frequencies 4 GHz˜6 GHz is input (see FIG. 6A).
In N−-mode sweep, a sweep frequency fL which acts as a local signal for the mixer 200 during the first sweep ranges from F1+Fi=4.4 GHz˜F2+Fi=6.4 GHz. As a result of the sweep, in addition to spectrum data S1b and S1a of the input signal S1 which are obtained at two sweep frequencies of 4.6 GHz and 5.4 GHz, measured data S2 is obtained. It will be seen that data S1b at 4.6 GHz is obtained from 5 GHz(=Fin)−4.6 GHz(fL)=0.4 GHz and the measured frequency fm is equal to 4.2 GHz, which is lower than the frequency 5 MHz of the input signal S1 by an amount which is equal to twice the intermediate frequency Fi=0.4 GHz or 0.8 GHz, and represents image data of the input signal S1. Data S1a is based on 5.4 GHz(=fL)−5 GHz(Fin)=0.4 GHz or is based on a correct input signal S1 of 5 GHz.
S2a results from 6.6 GHz(=fs)−6.2 GHz(=fL)=0.4 GHz, and appears at a measured frequency point of 5.8 GHz. In other words, an image of the input signal S2 has appeared as data S2a. In the second sweep, the sweep frequency fL ranges F1−Fi=3.6 GHz˜F2−Fi=5.6 GHz, as shown in FIG. 6C, and as a result of this sweep, spectrum data S1c, S1d of the first input signal S1 are obtained at two locations of the sweep frequency of 4.6 GHz and 5.4 GHz. It will be seen that data S1c at 4.6 GHz results from 5 GHz(=Fin)−4.6 GHz(=fL)=0.4 GHz or is based on the input signal S1 of 5 GHz while data S1d at 5.4 GHz results from 5.4 GHz(=fL)−5 GHz(=Fin)=0.4 GHz, and its measured frequency fm will be equal to 5.8 GHz which is higher than the frequency 5 GHz of the input signal S1 by an amount equal to twice Fi=0.4 GHz, representing image data of the input signal S1.
Accordingly, when the image eliminating processing mentioned above is applied to the first measured data (FIG. 6B) and the second measured data (FIG. 6C), it follows that at the measured frequency point of 5.8 GHz, a smaller one of data S2a and S1d, or data S2a is elected, and a consequence of the image eliminating processing becomes as shown in FIG. 6D where it will be noted that image data S2a of the input signal S2 which is located outside the range of set-up measured frequencies 4˜6 GHz remains unremoved.
Above problems are not limited to a spectrum analyzer, but also applies to a spurious measuring apparatus or any other frequency conversion sweeping measuring apparatus in which a signal being measured is frequency converted in a mixer using a frequency sweep signal for purpose of measurement.
It is an object of the present invention to provide a method of measurement by sweeping frequency conversion which enables a correct measurement without the provision of a preselector while avoiding an influence of image signals if measured data for a plurality of signals occur at a same measured frequency point.